The amplification degrees of PMT's, which are used in a detector, such as a scintillation camera, for detecting the position of a radioactive ray, become unequal to each other when these amplification degrees change with the passage of time. The deterioration of spatial resolution and energy resolution also results from such a change in the amplification degrees with the passage of time. To cope with these disadvantages, the following countermeasures have been proposed:
The first of them (Japanese notified patent application No. 55-19511 and Japanese laid-open patent application No. 58-9082) proposes that a reference light source such as a light emitting diode, which is to emit reference light, should be provided in a light guide so that an output may be taken from PMT's on the basis of the reference light and the high voltage applied to the PMT's may be adjusted on the basis of this output so as to stabilize the amplification degrees of the PMT's.
The second countermeasure (Japanese laid-open patent application No. 57-59184) proposes that a radiation source be allowed to lie on the central axis of one PMT after another in order to collect data for use in the correction of amplification degrees which have undergone a variation.
The third countermeasure (Japanese laid-open patent application No. 59-143981) proposes that a change in the amplification degrees of PMT's be found from an energy signal taken from the scintillation camera.
A fourth proposal by the present applicant in a prior application (i.e., in Japanese laid-open patent application No. 61-50088), is characterized in that a reference output is predetermined for the output of each PMT and that, when a scintillation occurs in a certain position, a reference output which ought to be taken from a PMT disposed closest to that position is compared with an actual output in order to obtain data on a change in the amplification degree of the PMT.
The first proposal has the disadvantage that the reference light per se, which is to be emitted by the reference light source is liable to be affected by a temperature change, secular change, etc.
The second proposal has the disadvantage that complicated work must be done if data is to be manually collected, while jigs are required if data is to be automatically collected. In either case, it takes much time to collect the data.
The third proposal has the disadvantage that data acquisition alternating with correction has to be repeated and that data obtained may not converge in some cases.
Although the fourth proposal is superior to the first to third proposals in that the defects of the latter are corrected by the former, the fourth proposal still leaves room for improvement in that many circuits are required while at best a low counting rate can be obtained.
It is therefore an object of the present invention to provide new concepts for solving the problems incidental to the prior art. In other words, it is an object of the present invention to provide an apparatus for stabilizing the amplification degrees of PMT's, which apparatus requires no reference light source for automatically correcting a secular change in the amplification degrees of PMT's to thereby stabilize various characteristics, and which requires a comparatively small number of circuits for collecting data on the amplification degrees of PMT's and yet the data acquisitions for the PMT's can be carried out in parallel with each other so that correction may be effected in a short time.